In recent years, light-emitting elements using electroluminescence have been actively researched and developed. As a basic structure of these light-emitting elements, a layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission can be obtained from the light-emitting substance.
Since such a light-emitting element is a self-luminous type, it has advantages over a liquid crystal display element, such as high visibility of the pixels and no need of backlight and is considered suitable for a flat panel display element. In addition, such a light-emitting element can be manufactured to be thin and light-weight, which is also a great advantage. Further, extremely high response speed is also a feature thereof.
Furthermore, since such a light-emitting element can be formed into a film form, planar light emission can be easily obtained by forming a large-area element. It is difficult to obtain this characteristic by using a point light source typified by an incandescent lamp or an LED or by using a line light source typified by a fluorescent lamp. Therefore, the light-emitting element described above also has a high utility value as a planar light source which is applicable to lighting or the like.
Such light-emitting elements using electroluminescence are broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. When an organic compound is used for a light-emitting substance, electrons and holes are injected into a layer containing a light-emitting organic compound from a pair of electrodes by applying voltage to a light-emitting element, and then a current flows therethrough. Then, by recombination of these carriers (electrons and holes), the light-emitting organic compound forms an excited state, and emits light when the excited state returns to a ground state.
With such a mechanism, such a light-emitting element is referred to as a current-excitation light-emitting element. Note that an excited state of an organic compound can be a singlet excited state or a triplet excited state. Light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence.
In improving element characteristics of such a light-emitting element, there are a lot of problems which depend on a substance, and in order to solve the problems, improvement of an element structure, development of a substance, and the like have been carried out (e.g., Non-Patent Document 1: Meng-Huan Ho, Yao-Shan Wu and Chin H. Chen, 2005 SID International Symposium Digest of Technical Papers, Vol. XXXVI. pp. 802-805).
In the light-emitting element described in Non-Patent Document 1, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) is used as a layer in contact with a light-emitting layer. However, NPB has low singlet excitation energy, and there is a possibility that the energy might be transferred from the light-emitting material in the excited state. Since the energy level of an excited state is particularly high in the case of a light-emitting material which emits blue light having a short wavelength, there is a higher possibility that the energy is transferred to NPB. There has been a problem that luminous efficiency of the light-emitting element is lowered due to transfer of the energy to NPB.